1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a sheet.
2. Description of the Related Art
As a conventional image forming apparatus such as a copying machine, a printer, or a facsimile, there is one having a structure in which a toner image formed on a photosensitive drum is primarily transferred onto an endless intermediate transfer belt and is then secondarily transferred onto the sheet. In addition, a sheet transport apparatus for transporting the sheet having the toner image secondarily transferred thereto to a fixing portion is provided between a secondary transfer portion for secondarily transferring the toner image onto the sheet, and the fixing portion for fixing the toner image onto the sheet.
As the sheet transport apparatus, there is one which includes a conveyor belt, a roller for driving the conveyor belt, and a fan for attaching the sheet to the conveyor belt, and transports the sheet having the toner image transferred thereto to the fixing portion while adsorbing the sheet (for example, see Japanese Patent Application Laid-Open No. 2005-316184).
FIG. 8 is a diagram illustrating a structure of a related sheet transport apparatus. A sheet transport apparatus 200 includes an endless sheet conveyor belt 201 which is suspended over multiple rollers 203 and has flexibility, and a fan 202 provided inside the sheet conveyor belt, for attracting a sheet.
In FIG. 8, a toner image formed on a photosensitive drum (not shown) is primarily transferred onto an intermediate transfer belt 211, and a second transfer portion 210 transfers a toner image T formed on the intermediate transfer belt 211 onto the sheet.
In a case where the sheet passing through the secondary transfer portion 210 and having the toner image secondarily transferred thereto is transported to a fixing portion (not shown), the sheet conveyor belt 201 is rotated in a direction of the fixing portion while the sheet is adsorbed to the sheet conveyor belt 201 by the fan 202. When a full-color image is formed, the intermediate transfer belt 211 is first rotated multiple times, and then the toner images T for each color are sequentially superposed on the intermediate transfer belt, thereby forming a full-color toner image T. After that, the full-color toner image T is secondarily transferred onto the sheet from the intermediate transfer belt 211.
In order to reliably adsorb and transport the sheet in the sheet transport apparatus and the image forming apparatus, it is necessary to increase an adsorbability (suction force) of the fan 202. However, in a case where the adsorbability of the fan 202 is increased, an air flow from the sheet conveyor belt 201 to the fan 202 is generated.
Particularly in a case of forming a full-color image, when the toner images T sequentially superposed on the intermediate transfer belt 211 reach a position at which the toner images T face the fan 202 before the toner images T are transferred onto the sheet as illustrated in FIG. 8, the toner images T may be scattered from the intermediate transfer belt 211 by the air flow. When the toner images T are scattered from the intermediate transfer belt 211, a sheet transport path may be contaminated, and the sheet is also contaminated when the sheet is transported. When the toner images T are scattered from the intermediate transfer belt 211, an image quality is degraded.
Japanese Patent Application Laid-Open No. H08-310686 discloses a structure in which a fan for adsorbing a sheet to a conveyor belt is stopped/driven according to a position of the sheet to be transported. In the structure disclosed in Japanese Patent Application Laid-Open No. H08-310686, noise due to a change of the attraction, which is caused when a trailing edge of the sheet passes through an adsorbing portion, is prevented from generating by stopping a sheet adsorbing operation before the trailing edge of the sheet passes through an adsorbing unit. In the structure disclosed in Japanese Patent Application Laid-Open No. H08-310686, the above-mentioned problem in that the toner image is scattered when the toner image formed on an image bearing member faces the conveyor belt is not taken into consideration. In the structure disclosed in Japanese Patent Application Laid-Open No. 08-310686, the toner image developed on the photosensitive drum serving as the image bearing member is immediately transferred onto the sheet by the transfer portion. Accordingly, the toner image formed on the photosensitive drum is not in a state of facing the conveyor belt.
Japanese Patent Application Laid-Open No. H08-110738 discloses a structure in which a fan for ventilation of the apparatus main body is turned up during a time when formation of an image on the sheet is not performed, and is turned down during image formation. In a case of operating the fan for ventilation during image formation, the fan can be driven with only a small power which does not affect the image formation. Accordingly, in the structure disclosed in Japanese Patent Application Laid-Open No. H08-110738, in consideration of the fact that the ventilation by the fan for ventilation when the fan is insufficient with such a small power that does not affect the image formation, the exhaust fan is strongly driven during a time when the image formation is not performed. In the structure disclosed in Japanese Patent Application Laid-Open No. H08-110738, the above-mentioned problem in that the toner image is scattered when the toner image formed on the image bearing member faces the conveyor belt is not taken into consideration. In the structure disclosed in Japanese Patent Application Laid-Open No. H08-110738, because the toner image developed on the photosensitive drum serving as the image bearing member is immediately transferred onto the sheet by the transfer portion, the toner image formed on the photosensitive drum is not in a state of facing the conveyor belt.
Japanese Patent Application Laid-Open No. 2002-258722 discloses a structure in which disturbance of toner particles on a sheet is prevented by turning down or stopping a cooling fan which is used for preventing temperature rise within the apparatus due to heat generated from the fixing device when the leading edge of the sheet having an image formed thereon is present between the transfer portion and the fixing device. In the structure disclosed in Japanese Patent Application Laid-Open No. 2002-258722, an effect on the toner on the sheet due to the fan for preventing temperature rise within the apparatus is taken into consideration. However, the problem in that the toner image is scattered when the toner image formed on the image bearing member faces the conveyor belt is not taken into consideration.